Which brand of Thai curry paste are you using? Mae Ploy has shrimp paste in it, whereas Arroy-D is vegetarian. Nothing wrong with that of course, but if you're stocked with a large supply of Arroy-D, then consider getting a little tub of shrimp paste to doctor it up.
And ditto on all the herbs and aromatics others have mentioned. Even better would be getting some key limes instead of regular ones.
If it's for a bonafide Thai curry dish, then also start with a good chicken stock made with bones, mirepoix, fresh turmeric root, lemongrass, lime leaves, ginger, galangal, and white peppercorns.
Pounding Thai bird chilies in a mortar will make them extra spicy. Another option would be to make prik nam pla on the side (birds, fish sauce and lime juice) for those customers who want it hotter and more pungent.
Lastly, make sure your fish sauce doesn't suck. :D The ingredient label should just be fish, salt and water. And maybe sugar but that's it. If it says hydrolized proteins or anything like that, then it was made with a sped up fermentation process, and it won't taste as good.
And even among the pure brands, the salt content will be very different, so taste first and adjust accordingly. It gets saltier with longer cooking time.